Avoid an overflow in the ELF SYSV ABI hash function.

Avoid an overflow in the ELF SYSV ABI hash function.
The hash function is supposed to return a value less than or equal
to 0x0fffffff.  Due to a bug in the sample code supplied with the
ELF SYSV ABI documentation, the hash function can overflow on 64-bit
systems.  Apply the same fix used by GNU libc, MUSL libc and FreeBSD.
Prompted by https://maskray.me/blog/2023-04-12-elf-hash-function
OK tb@ miod@