Micro-optimize serializing class names

[TysonAndre]

Because of the memcpy, compilers can't infer that ZSTR_LEN (i.e. class_name->len)
did not change, so they copy it out of memory into a register for the last two
accesses. (i.e. mov rdx,QWORD PTR [r15+0x10] is repeated)
php_var_serialize_string already does something similar.

(register allocation, instructions, etc also changed elsewhere because of the code change)

# Old
/path/to/php-src/ext/standard/var.c:744
     b2d:	4c 8b 0c 24          	mov    r9,QWORD PTR [rsp]
memcpy():
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
     b31:	41 bb 3a 22 00 00    	mov    r11d,0x223a
     b37:	4c 89 f6             	mov    rsi,r14
     b3a:	48 89 c1             	mov    rcx,rax
php_var_serialize_class_name():
/path/to/php-src/ext/standard/var.c:744
     b3d:	4c 01 c9             	add    rcx,r9
memcpy():
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
     b40:	66 44 89 19          	mov    WORD PTR [rcx],r11w
php_var_serialize_class_name():
/path/to/php-src/ext/standard/var.c:746
     b44:	48 83 c1 02          	add    rcx,0x2
memcpy():
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
     b48:	49 8b 57 10          	mov    rdx,QWORD PTR [r15+0x10]
     b4c:	48 89 cf             	mov    rdi,rcx
     b4f:	e8 00 00 00 00       	call   b54 <php_var_serialize_intern.part.0+0x644>
php_var_serialize_class_name():
/path/to/php-src/ext/standard/var.c:748
     b54:	49 8b 4f 10          	mov    rcx,QWORD PTR [r15+0x10]
     b58:	48 01 c1             	add    rcx,rax
memcpy():
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
     b5b:	66 44 89 29          	mov    WORD PTR [rcx],r13w


#new

php_var_serialize_class_name():
/path/to/php-src/ext/standard/var.c:744
     b2e:	48 8b 14 24          	mov    rdx,QWORD PTR [rsp]
memcpy():
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
     b32:	41 ba 3a 22 00 00    	mov    r10d,0x223a
     b38:	4c 8b 4c 24 08       	mov    r9,QWORD PTR [rsp+0x8]
     b3d:	48 89 c1             	mov    rcx,rax
php_var_serialize_class_name():
/path/to/php-src/ext/standard/var.c:744
     b40:	48 01 d1             	add    rcx,rdx
memcpy():
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
     b43:	4c 89 ce             	mov    rsi,r9
     b46:	4c 89 f2             	mov    rdx,r14
     b49:	66 44 89 11          	mov    WORD PTR [rcx],r10w
php_var_serialize_class_name():
/path/to/php-src/ext/standard/var.c:746
     b4d:	48 83 c1 02          	add    rcx,0x2
memcpy():
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
     b51:	48 89 cf             	mov    rdi,rcx
     b54:	e8 00 00 00 00       	call   b59 <php_var_serialize_intern.part.0+0x649>
     b59:	41 bb 22 3a 00 00    	mov    r11d,0x3a22
     b5f:	66 46 89 1c 30       	mov    WORD PTR [rax+r14*1],r11w