[xc7] Support RAMB36E1

[litghost]

Tasks:

• [x] Ensure that RAMB36E1 can be placed and routed
• [x] Add simulation model for RAMB18E1 and RAMB36E1 Xilinx Unisims now exist!
• [ ] Create test benches to verify pre- and post-place and routed designs using RAMB18E1 and RAMB36E1
• [ ] Verify RAMB18E1 and RAMB36E1 operation in hardware

Support not required for:

• RAMB36E1 1-bit cascades
• ECC support

[litghost]

RAMB18/36 tested on hardware:

• [ ] RAMB18 width = 2
• [ ] RAMB18 width = 4
• [ ] RAMB18 width = 9
• [x] RAMB18 width = 18
• [x] RAMB18 width = 36
• [ ] RAMB36 width = 1
• [ ] RAMB36 width = 2
• [ ] RAMB36 width = 4
• [ ] RAMB36 width = 9
• [ ] RAMB36 width = 18
• [x] RAMB36 width = 36
• [x] RAMB36 width = 72