apply hidden visibility to internal math functions

this makes significant differences to codegen on archs with an
expensive PLT-calling ABI; on i386 and gcc 7.3 for example, the sin
and sinf functions no longer touch call-saved registers or the stack
except for pushing outgoing arguments. performance is likely improved
too, but no measurements were taken.

src/internal/libm.h

@@ -167,33 +167,33 @@ do {                                              \
 
 /* fdlibm kernel functions */
 
-int    __rem_pio2_large(double*,double*,int,int,int);
-
-int    __rem_pio2(double,double*);
-double __sin(double,double,int);
-double __cos(double,double);
-double __tan(double,double,int);
-double __expo2(double);
-double complex __ldexp_cexp(double complex,int);
-
-int    __rem_pio2f(float,double*);
-float  __sindf(double);
-float  __cosdf(double);
-float  __tandf(double,int);
-float  __expo2f(float);
-float complex __ldexp_cexpf(float complex,int);
-
-int __rem_pio2l(long double, long double *);
-long double __sinl(long double, long double, int);
-long double __cosl(long double, long double);
-long double __tanl(long double, long double, int);
+hidden int    __rem_pio2_large(double*,double*,int,int,int);
+
+hidden int    __rem_pio2(double,double*);
+hidden double __sin(double,double,int);
+hidden double __cos(double,double);
+hidden double __tan(double,double,int);
+hidden double __expo2(double);
+hidden double complex __ldexp_cexp(double complex,int);
+
+hidden int    __rem_pio2f(float,double*);
+hidden float  __sindf(double);
+hidden float  __cosdf(double);
+hidden float  __tandf(double,int);
+hidden float  __expo2f(float);
+hidden float complex __ldexp_cexpf(float complex,int);
+
+hidden int __rem_pio2l(long double, long double *);
+hidden long double __sinl(long double, long double, int);
+hidden long double __cosl(long double, long double);
+hidden long double __tanl(long double, long double, int);
 
 /* polynomial evaluation */
-long double __polevll(long double, const long double *, int);
-long double __p1evll(long double, const long double *, int);
+hidden long double __polevll(long double, const long double *, int);
+hidden long double __p1evll(long double, const long double *, int);
 
 extern int __signgam;
-double __lgamma_r(double, int *);
-float __lgammaf_r(float, int *);
+hidden double __lgamma_r(double, int *);
+hidden float __lgammaf_r(float, int *);
 
 #endif

src/math/__invtrigl.h

@@ -1,6 +1,6 @@
 /* shared by acosl, asinl and atan2l */
 #define pio2_hi __pio2_hi
 #define pio2_lo __pio2_lo
-extern const long double pio2_hi, pio2_lo;
+hidden extern const long double pio2_hi, pio2_lo;
 
-long double __invtrigl_R(long double z);
+hidden long double __invtrigl_R(long double z);