sandbox: Remove a TEXTREL on Linux ARM.

sandbox: Remove a TEXTREL on Linux ARM.

On Android, the sandbox logic is put into a shared library and the linker errors
out on having a TEXTREL.

The LDR pseudo-instruction directs the assembler to place the address of the
label in a "literal pool"[1], and then use a PC-relative load from that pool to
get the address during execution. The pool is responsible for generating
the R_ARM_RELATIVE TEXTREL.

Using the ADR instruction[2] does not produce the TEXTREL. This instruction
directs the assembler to calculate the PC-relative address to the label using an
immediate.

The text makes this difference more clear:

...
   8:   4805            ldr     r0, [pc, #20]   ; (20 <SyscallAsm+0x20>)
   a:   e007            b.n     1c <SyscallAsm+0x1c>
...
  1c:   bd80            pop     {r7, pc}
  1e:   0000            .short  0x0000
  20:   0000001c        .word   0x0000001c

Versus:

...
   8:   4804            ldr     r0, [pc, #16]   ; (1c <SyscallAsm+0x1c>)
   a:   e007            b.n     1c <SyscallAsm+0x1c>
...
  1c:   bd80            pop     {r7, pc}
  1e:   bf00            nop


[1] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0041c/Babbfdih.html
[2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0040d/Cihdhgbe.html

BUG=308763
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=242141

[jln (very slow on Chromium), 2013-12-19 23:38:42]

lgtm

Nice bug hunting!

[commit-bot: I haz the power, 2013-12-20 15:46:41]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rsesek@chromium.org/119453002/1

[commit-bot: I haz the power, 2013-12-20 17:48:33]

Retried try job too often on win_rel for step(s) unit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[commit-bot: I haz the power, 2013-12-20 17:51:15]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rsesek@chromium.org/119453002/1

[commit-bot: I haz the power, 2013-12-20 19:35:02]

Change committed as 242141