Fix __VERSIONER_NO_GUARD cases for availability.

libc++ still depends on these being declared even if they are
unavailable. This results in a worse error message (a link error
rather than a compiler diagnostic) if these functions end up being
used, but without the decl headers like libc++'s math.h can't be
included because it refers to an undeclared function.

For the cases where weak symbols aren't being used, don't annotate
these functions with their availability information.

Also need to avoid using __builtin_available for this case because the
NDK doesn't have __isOSVersionAtLeast yet.

__ANDROID_UNGUARDED_AVAILABILITY__ is being used as a proxy for
"building for the NDK" here because we don't have a good signal for
that which works for both the NDK proper and the NDK-in-the-platform
case.

Test: imported into the NDK, built and tested NDK
Bug: None
Change-Id: I9ef3e19a8fa083bca0be47b80dfef7ba52a94866
diff --git a/libc/include/math.h b/libc/include/math.h
index 8c084b2..040dc96 100644
--- a/libc/include/math.h
+++ b/libc/include/math.h
@@ -178,9 +178,9 @@
 long double scalbnl(long double __x, int __exponent) __RENAME_LDBL(scalbn, 3, 3);
 
 /* TODO: once the NDK only supports >= 18, use __RENAME_LDBL here too. */
-double scalbln(double __x, long __exponent) __INTRODUCED_IN_X86(18) __VERSIONER_NO_GUARD;
-float scalblnf(float __x, long __exponent) __INTRODUCED_IN_X86(18) __VERSIONER_NO_GUARD;
-long double scalblnl(long double __x, long __exponent) __INTRODUCED_IN_X86(18) __VERSIONER_NO_GUARD;
+double scalbln(double __x, long __exponent) __INTRODUCED_IN_X86_NO_GUARD_FOR_NDK(18);
+float scalblnf(float __x, long __exponent) __INTRODUCED_IN_X86_NO_GUARD_FOR_NDK(18);
+long double scalblnl(long double __x, long __exponent) __INTRODUCED_IN_X86_NO_GUARD_FOR_NDK(18);
 
 double cbrt(double __x);
 float cbrtf(float __x);
@@ -280,11 +280,11 @@
 
 double nextafter(double __x, double __y);
 float nextafterf(float __x, float __y);
-long double nextafterl(long double __x, long double __y) __RENAME_LDBL(nextafter, 3, 21) __VERSIONER_NO_GUARD;
+long double nextafterl(long double __x, long double __y) __RENAME_LDBL_NO_GUARD_FOR_NDK(nextafter, 3, 21);
 
-double nexttoward(double __x, long double __y) __INTRODUCED_IN(18) __VERSIONER_NO_GUARD;
+double nexttoward(double __x, long double __y) __INTRODUCED_IN_NO_GUARD_FOR_NDK(18);
 float nexttowardf(float __x, long double __y);
-long double nexttowardl(long double __x, long double __y) __RENAME_LDBL(nexttoward, 18, 18) __VERSIONER_NO_GUARD;
+long double nexttowardl(long double __x, long double __y) __RENAME_LDBL_NO_GUARD_FOR_NDK(nexttoward, 18, 18);
 
 double fdim(double __x, double __y);
 float fdimf(float __x, float __y);
@@ -300,7 +300,7 @@
 
 double fma(double __x, double __y, double __z);
 float fmaf(float __x, float __y, float __z);
-long double fmal(long double __x, long double __y, long double __z) __RENAME_LDBL(fma, 3, 21) __VERSIONER_NO_GUARD;
+long double fmal(long double __x, long double __y, long double __z) __RENAME_LDBL_NO_GUARD_FOR_NDK(fma, 3, 21);
 
 #define isgreater(x, y) __builtin_isgreater((x), (y))
 #define isgreaterequal(x, y) __builtin_isgreaterequal((x), (y))