tree-wide: Replace exp10() with our own impl

exp10() has a symbol version > 2.34 on latest glibc. To allow 
dropping our baseline required glibc runtime version to <= 2.34, 
let's add our own version to prevent pulling in the newer symbol 
from glibc.

src/basic/meson.build

@@ -64,6 +64,7 @@ basic_sources = files(
         'log.c',
         'log-context.c',
         'login-util.c',
+        'math-util.c',
         'memfd-util.c',
         'memory-util.c',
         'mempool.c',