Documentation: Additional optimization levels for ARM/Linux (dotnet/coreclr#6548)

Documentation: Additional optimization levels for ARM/Linux (dotnet/coreclr#6548)

This instruction is to enable -Ofast or -Oz optimization level of clang on ARM/Linux.
Below is file-size comparison result of major components among -O3, -Ofast,
and -Oz on ARM/Linux-based Raspberry Pi2 board (Ubuntu/ARM 14.04 32bit).

* Linux/ARM: Release Build + O3(default): stripped , commit: dotnet/coreclr@561b64d2c210b4d999de7f1ac55756704eaba784 (Jul-26-2016)
------------------------------------------------------------------------------------------------------------------
-rwxr-xr-x 1 leemgs leemgs     18,472 Jul 27 16:14 corerun
-rwxr-xr-x 1 leemgs leemgs  1,512,828 Jul 27 18:33 libclrjit.so
-rwxr-xr-x 1 leemgs leemgs  4,525,328 Jul 27 18:33 libcoreclr.so
-rwxr--r-- 1 leemgs leemgs     29,696 Jul 27 00:55 ./mscorlib.dll
-rwxr--r-- 1 leemgs leemgs  2,211,840 Jul 27 00:54 ./System.Private.CoreLib.dll
Total:   8,298,164 bytes

* Linux/ARM: Release Build + Ofast: stripped , commit: dotnet/coreclr@561b64d2c210b4d999de7f1ac55756704eaba784 (Jul-26-2016)
-------------------------------------------------------------------------------------------------------------
-rwxr-xr-x 1 leemgs leemgs    18,496 Jul 26 05:00 ./corerun
-rwxr-xr-x 1 leemgs leemgs 1,512,844 Jul 26 04:59 ./libclrjit.so
-rwxr-xr-x 1 leemgs leemgs 4,525,360 Jul 26 04:59 ./libcoreclr.so
-rwxr--r-- 1 leemgs leemgs    29,696 Jul 26 00:49 ./mscorlib.dll
-rwxr--r-- 1 leemgs leemgs 2,211,840 Jul 26 00:48 ./System.Private.CoreLib.dll
Total:   8,298,236 bytes

* Linux/ARM: Release Build + Oz: stripped , commit: dotnet/coreclr@561b64d2c210b4d999de7f1ac55756704eaba784 (Jul-26-2016)
---------------------------------------------------------------------------------------------------------
-rwxr-xr-x 1 leemgs leemgs    14,400 Jul 29 05:02 ./corerun
-rwxr-xr-x 1 leemgs leemgs 1,181,160 Jul 29 04:40 ./libclrjit.so
-rwxr-xr-x 1 leemgs leemgs 3,407,200 Jul 29 04:40 ./libcoreclr.so
-rwxr--r-- 1 leemgs leemgs    29,696 Jul 29 00:49 ./mscorlib.dll
-rwxr--r-- 1 leemgs leemgs 2,211,840 Jul 29 00:48 ./System.Private.CoreLib.dll
Total:  6,844,296 bytes

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>
Commit migrated from https://github.com/dotnet/coreclr/commit/22c9f9e5b36eccde23d4a2187eef8f7985aed4b3