perl: install libperl.so using $(INSTALL_BIN) to enable stripping

perl: install libperl.so using $(INSTALL_BIN) to enable stripping

Install libperl.so with executable permissions (0755) instead of copying
it as-is.

This allows the OpenWrt build system to recognize the library as a binary and
automatically run the "strip" utility on it. Stripping removes unnecessary
debugging metadata and helper symbols that are only needed for development but
not for running Perl on the router.

This change reduces the installed size of libperl.so on the device:
- Before (not stripped): 3.7 MB (3,929,652 bytes)

```
root@turris:~# file /usr/lib/perl5/5.40/CORE/libperl.so
/usr/lib/perl5/5.40/CORE/libperl.so: ELF 32-bit MSB shared object, PowerPC or cisco 4500, version 1 (SYSV), dynamically linked, with debug_info, not stripped
```

- After (stripped):     3.5 MB (3,674,081 bytes)

```
root@turris:~# file /usr/lib/perl5/5.40/CORE/libperl.so
/usr/lib/perl5/5.40/CORE/libperl.so: ELF 32-bit MSB shared object, PowerPC or cisco 4500, version 1 (SYSV), dynamically linked, no section header
```

Saving ~255 KB of flash storage space on target devices.

Signed-off-by: Josef Schlehofer <redacted>