Mark __aeabi_lmul section as allocatable and executable (#722)

When using a linker script which does not provide a .time_critical.
section that is marked "ax" and wrongly define
PICO_INT64_OPS_IN_RAM=1 this would generate a malformed
executable with strange runtime bugs in the worst case.

src/rp2_common/pico_int64_ops/pico_int64_ops_aeabi.S

@@ -10,11 +10,16 @@
 
 #include "pico/asm_helper.S"
 
+.macro int64_section name
 #if PICO_INT64_OPS_IN_RAM
-.section RAM_SECTION_NAME(__aeabi_lmul)
+.section RAM_SECTION_NAME(\name), "ax"
 #else
-.section SECTION_NAME(__aeabi_lmul)
+.section SECTION_NAME(\name), "ax"
 #endif
+.endm
+
+int64_section __aeabi_lmul
+
 wrapper_func __aeabi_lmul
     muls   r1, r2
     muls   r3, r0
@@ -41,4 +46,3 @@ wrapper_func __aeabi_lmul
     adcs   r1, r2
     add    r1, r12
     bx lr
-