gcc: fix uclibc runtime issue with gcc-8 for xtensa

gcc: fix uclibc runtime issue with gcc-8 for xtensa

gcc-8.1 for xtensa miscompiles uClibc dynamic linker due to gcc PR
target/65416. The build completes successfully, but the binary is
non-functional because the following fragment in the _dl_get_ready_to_run
in ld-uClibc.so overwrites register spill area on stack causing register
corruption in the previous call frame and a subsequent crash:

    419f:       f0c1b2          addi    a11, a1, -16
    41a2:       1ba9            s32i.n  a10, a11, 4
    41a4:       0bc9            s32i.n  a12, a11, 0
    41a6:       5127f2          l32i    a15, a7, 0x144
    41a9:       1765b2          s32i    a11, a5, 92
    41ac:       4e2782          l32i    a8, a7, 0x138
    41af:       146af2          s32i    a15, a10, 80
    41b2:       001b10          movsp   a1, a11

The crash terminates the init process and causes kernel panic.
The fix prevents reordering of movsp opcode and any access to the stack
frame memory and is applicable to all existing gcc versions.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>