<math.h>: more use of __RENAME_LDBL.

Now the NDK doesn't support API levels below 21, we don't actually need
weird x86-specific annotations. In general we haven't been removing
this historical information because it might be interesting to someone,
and there's no real reason to remove it, but we've had versioner bugs
recently with these more complex cases.

Test: treehugger
Change-Id: Ia457bb338ecf55af8e319e411ec3bf48a03f3c03
diff --git a/libc/include/math.h b/libc/include/math.h
index a5fa7c3..d3b4a33 100644
--- a/libc/include/math.h
+++ b/libc/include/math.h
@@ -177,10 +177,9 @@
 float scalbnf(float __x, int __exponent);
 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_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 scalbln(double __x, long __exponent);
+float scalblnf(float __x, long __exponent);
+long double scalblnl(long double __x, long __exponent) __RENAME_LDBL(scalbln, 9, 18);
 
 double cbrt(double __x);
 float cbrtf(float __x);
@@ -282,7 +281,7 @@
 float nextafterf(float __x, float __y);
 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_NO_GUARD_FOR_NDK(18);
+double nexttoward(double __x, long double __y);
 float nexttowardf(float __x, long double __y);
 long double nexttowardl(long double __x, long double __y) __RENAME_LDBL_NO_GUARD_FOR_NDK(nexttoward, 18, 18);