[PATCH][ARM]Honour __heap_limit__ during _sbrk() memory allocation checking.

[Renlin Li]

Hi all,

HeapInfo is returned when SYS_HEAPINFO Angel semihost call is triggered, 
the following data will be returned.

struct block {
     int heap_base;
     int heap_limit;
     int stack_base;
     int stack_limit;
};

the heap_limit should be honoured while doing dynamic memory allocation. 
Currently, _sbrk() does a simple check by comparing the heap end against 
stack pointer. This is not enough, if __heap_limit is valid, it also 
should be used to predicate whether enough heap memory is there to be 
allocated.

libstdc++v3, gcc regression tests are checked using arm-none-eabi 
toolchain, on new issues.

Okay to commit?

Actually, aarch64 target also has the same problem. In addition, 
__heap_base__ is not used either. I will come up with a patch later.

libgloss/ChangeLog:

2015-02-13  Renlin Li  <renlin.li@arm.com>

     * arm/crt0.S: Initialise __heap_limit when ARM_RDI_MONITOR is defined.
     * arm/syscalls.c: define __heap_limit global symbol.
     * arm/syscalls.c (_sbrk): Honour __heap_limit.
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