build: run android builder with android-L devices.

[hyangah]

The current builder cross-compiles the test binaries, runs the compiled binaries on the device, and parses the output of them. This does not work for android-L that requires PIE and refuses to run the test binaries with the following error message.

"only position independent executables (PIE) are supported."

Probably related to : #6940

[minux]

Does Android L support PIE executable with text relocations?

We can easily generate PIE executable if text relocation is supported.
External linking with PIE (#6940) is a different story though.
(Removing text relocations is ##9210).

To eliminate text relocations, we need to modify the way we load
addresses into registers. Right now, we use constant pool entry
for absolute address, to support PIE (or PIC), we have to use PC
relative load.

MOVW addr(SB), R0 is currently translated to:
ldr r11, [pc, #cpoff]; constant pool uses R_ADDR reloc.
ldr r0, [r11]

To switch to PIC, it needs to be translated to:
ldr r11, [pc, #cpoff]; constant pool uses R_PCREL+addend
ldr r0, [pc, r11]

MOVW $addr(SB), R1 is currently translated to:
ldr r1, [pc, #cpoff]
To switch to PIC, it needs to be:
ldr r11, [pc, #cpoff]
add r11, pc, r1

[quarnster]

Just adding -ldflags="-extldflags=-pie" works on my Nexus 6. While it complains about the text relocations, it appears to work just fine:
W/linker (12596): /data/local/tmp/test has text relocations. This is wasting memory and prevents security hardening. Please fix.

[crawshaw]

@quarnster huh, that didn't work last time I tried it, but you're right.

[gopherbot]

CL https://golang.org/cl/16055 mentions this issue.

[gopherbot]

CL https://golang.org/cl/16054 mentions this issue.

[gopherbot]

CL https://golang.org/cl/16114 mentions this issue.

[gopherbot]

CL https://golang.org/cl/16115 mentions this issue.

[gopherbot]

CL https://golang.org/cl/16116 mentions this issue.