Remove __INTRODUCED_IN for obsolete API levels.

The NDK only supports >= 16, so remove anything older than that to avoid
giving the misleading impression that such old targets are still
supported.

(This change doesn't touch <unistd.h>. I'll follow up with that once the
outstanding FORTIFY changes to that file are in.)

Test: builds
Change-Id: I6cc6ecdb99fe228a4afa71f78e5fd45309ba9786
diff --git a/libc/include/math.h b/libc/include/math.h
index beb66a7..7efc83a 100644
--- a/libc/include/math.h
+++ b/libc/include/math.h
@@ -217,7 +217,7 @@
 long double lgammal(long double __x) __RENAME_LDBL(lgamma, 3, 21);
 
 double tgamma(double __x);
-float tgammaf(float __x) __INTRODUCED_IN_ARM(13) __INTRODUCED_IN_MIPS(13) __INTRODUCED_IN_X86(9);
+float tgammaf(float __x);
 long double tgammal(long double __x) __RENAME_LDBL(tgamma, 3, 21);
 
 double ceil(double __x);
@@ -276,8 +276,8 @@
 float copysignf(float __value, float __sign) __attribute_const__;
 long double copysignl(long double __value, long double __sign) __RENAME_LDBL(copysign, 3, 3) __attribute_const__;
 
-double nan(const char* __kind) __attribute_const__ __INTRODUCED_IN_ARM(13) __INTRODUCED_IN_MIPS(13) __INTRODUCED_IN_X86(9);
-float nanf(const char* __kind) __attribute_const__ __INTRODUCED_IN_ARM(13) __INTRODUCED_IN_MIPS(13) __INTRODUCED_IN_X86(9);
+double nan(const char* __kind) __attribute_const__;
+float nanf(const char* __kind) __attribute_const__;
 long double nanl(const char* __kind) __RENAME_LDBL(nan, 13, 13) __attribute_const__;
 
 double nextafter(double __x, double __y);