https://bugs.gentoo.org/935529

__WORDSIZE__ is used for getting correct ABI struct sizes, its undeclared on musl unless you include the bits/reg.h header.

--- a/include/perfmon/pfmlib.h    2023-03-29 02:44:33.000000000 +0300
+++ b/include/perfmon/pfmlib.h  2024-08-24 12:45:29.084265290 +0300
@@ -38,6 +38,9 @@
 #include <unistd.h>
 #include <inttypes.h>
 #include <stdio.h>
+#ifndef __GLIBC__
+#include <bits/reg.h>
+#endif
 
 #define LIBPFM_VERSION         (4 << 16 | 0)
 #define PFM_MAJ_VERSION(v)     ((v)>>16)