Fix placement of code into RAM, enable gc-sections

[igrr]

Related to #856, #826, #803, #734.

• enable -gc-sections linker option (Add support for multiple progmem sections #734)
• enable -ffunction-sections -fdata-sections (in conjunction with the above, helps with Sketch too big... #826)
• fix ICACHE_RAM_ATTR ignored when used in .cpp file #856 by making ICACHE_RAM_ATTR place functions into .iram.text instead of .text. Place .iram.text into .text segment via linker script. This allows to (selectively) place code into RAM from .cpp files and moves code from .c files into .irom.text by default (.text' will not fit in region iram1_0_seg' with c modules #803).

Reduces size of the sketches somewhat.
Without this change (HelloServer sample sketch):

Sketch uses 349,510 bytes (33%) of program storage space. Maximum is 1,044,464 bytes.
Global variables use 45,674 bytes (55%) of dynamic memory, leaving 36,246 bytes for local variables. Maximum is 81,920 bytes.

With this change:

Sketch uses 225,310 bytes (21%) of program storage space. Maximum is 1,044,464 bytes.
Global variables use 36,506 bytes (44%) of dynamic memory, leaving 45,414 bytes for local variables. Maximum is 81,920 bytes.

Compile tests pass, but might break some sketches at run time. Needs testing.

[Juppit]

On 06.10.2015 at 01:23 wrote Ivan Grokhotkov:

|Sketch uses 349,510 bytes (33%) of program storage space. Maximum is
1,044,464 bytes. Global variables use 45,674 bytes (55%) of dynamic
memory, leaving 36,246 bytes for local variables. Maximum is 81,920 bytes. |

With this change:

|Sketch uses 225,310 bytes (21%) of program storage space. Maximum is
1,044,464 bytes. Global variables use 36,506 bytes (44%) of dynamic
memory, leaving 45,414 bytes for local variables. Maximum is 81,920 bytes. |

Compile tests pass, but might break some sketches at run time. Needs
testing.

Reduced my sketch size from 32% to 20% and RAM usage from 52% to 41%.
No problems occurred.
Great, once again.