Fix unbalanced diagnostics pragma

[aykevl]

Clang 9 is complaining about this and the code appears to be wrong.

In file included from /home/ayke/src/github.com/tinygo-org/tinygo/lib/picolibc/newlib/libm/common/s_expm1.c:145:
/home/ayke/src/github.com/tinygo-org/tinygo/lib/picolibc/newlib/libm/common/math_config.h:222:24: error: pragma diagnostic pop could not pop, no matching push [-Werror,-Wunknown-pragmas]
#pragma GCC diagnostic pop
                       ^
1 error generated.

Note that I'm using a custom build system (to avoid external dependencies, among other reasons) so perhaps there is something different in my setup that causes this error to appear.

---

Sidenote: thanks a lot for your work on picolibc! It's being used now as a C library in TinyGo for most bare metal targets: tinygo-org/tinygo#871

[keith-packard]

Thanks for the fix -- GCC doesn't even warn about this issue.