devel/electron*: Fix build on main

After src snapshot 0c47b9c211, the build began failing with:

../../third_party/nasm/include/compiler.h:249:21: error: static
declaration of 'mempcpy' follows non-static declaration

249 | static inline void *mempcpy(void *dst, const void *src, size_t n)
    |                     ^

/usr/include/string.h:71:8: note: previous declaration is here

71 | void    *(mempcpy)(void * __restrict, const void * __restrict, size_t);
   |           ^

• Restore a default definition of HAVE_MEMPCPY to indicate that mempcpy(3) is available. It has been part of FreeBSD since 13.1.

• Remove a patch used to detect mempcpy(3) in /usr/include/string.h. It is no longer necessary and does not detect the function after src snapshot 0c47b9c211.

Approved by: tagattie (maintainer)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46054