Fix SPM HAL test

[orenc17]

Description

When accessing non-secure ram and flash r1 was actually used by the calling function.
Change to a callee saved register.

Pull request type

[ ] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[X] Test update
[ ] Breaking change

@alzix @mikisch81 please review

[cmonr]

Waiting on #9131 for a fix to Travis CI.

[alzix]

@orenc17 please elaborate on the change.
What is the problem you experience with using r1 register?
You cannot use r4. The reason it is called calleesave is that the callee must restore it's value upon returning from the function. While caller save registers are backed up by the caller itself, so callee may use them as scratch registers.
Since call_mem is a naked function no epilogue is generated by the compiler, so no one will cleanup r4 register you are trying to use.

[orenc17]

@alzix call_mem() is being called by test_memory() which receives 2 arguments.
that means that r0 and r1 are being used.
so when call_mem(), which is a naked function as you said, is being called on the non-secure cases it will over-write the r1 register which holds the 2nd paramenter of test_memory() and that's what this PR is fixing

and anyway test_memory() takes care of saving r4 and restoring it

[alzix]

According to AAPCS:

A subroutine must preserve the contents of the registers r4-r8, r10, r11 and SP (and r9 in PCS variants that designate r9 as v6).

[alzix]

LGTM

[alzix]

created a ticket for GCC_ARM https://bugs.launchpad.net/gcc-arm-embedded/+bug/1809607

[cmonr]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 9 of 9 test jobs passed
Build number : 1
Build artifacts