php8: fix linking on riscv64 platform

php8: fix linking on riscv64 platform

The buildbots failed for the mentioned platform with the following error:
(I shortened the pathnames and broke long lines a little bit for readability)

.../lib/gcc/riscv64-openwrt-linux-musl/12.3.0/../../../../
  riscv64-openwrt-linux-musl/bin/ld: Zend/zend_execute_API.o: in function `.L533':
zend_execute_API.c:(.text+0x1b1c): undefined reference to `__atomic_exchange_1'
.../riscv64-openwrt-linux-musl/bin/ld: Zend/zend_atomic.o:
  in function `zend_atomic_bool_exchange':
zend_atomic.c:(.text+0xc): undefined reference to `__atomic_exchange_1'
collect2: error: ld returned 1 exit status
make[4]: *** [Makefile:350: sapi/cli/php] Error 1

Inspired by the blog post[1], linking to libatomic explicitly seems to
do the trick.

[1] A RISC-V gcc pitfall revealed by a glibc update
https://blog.jiejiss.com/A-RISC-V-gcc-pitfall-revealed-by-a-glibc-update

Signed-off-by: Michael Heimpold <mhei@heimpold.de>