esp32: Add support for building with external SPI RAM.

This patch adds support for building the firmware with external SPI RAM
enabled.  It is disabled by default because it adds overhead (due to
silicon workarounds) and reduces performance (because it's slower to have
bytecode and objects stored in external RAM).

To enable it, either use "make CONFIG_SPIRAM_SUPPORT=1", or add this line
to you custom makefile/GNUmakefile (before "include Makefile"):

    CONFIG_SPIRAM_SUPPORT = 1

When this option is enabled the MicroPython heap is automatically allocated
in external SPI RAM.

Thanks to Angus Gratton for help with the compiler and linker settings.

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